In order to ensure greater fuel efficiency and lower exhaust smoke and particulate matter (PM) emission, the maximum and medium injection pressures in modern diesels are constantly increased. By now, commonly used injection pressures have reached 1600-2000 Bar, and in the near future they will reach 2500 Bar and more. The increase of the maximum and medium injection pressures is facilitated by the increase in the nozzle needle lift and closing pressures (pressures causing the needle to start its travel upward and reverse, to travel downward and seat on the nozzle's cone). The latter is also especially important for lowering exhaust smoke emission, in particular (PM), because by increasing the force on the nozzles needle, the needle closes faster, resulting sharp EOI (nd Of Injection), thus reducing the quantity of the fuel injected into the combustion chamber under low pressure at the final phase of the injection.
In modern diesels, needle-spring locking devices are most frequently used, with a cylindrical helical spring usually disposed in a central cavity formed in the pump-injector) body. The diameter of such cavities in actual diesels does not exceed 12-14 mm, because larger diameters would not allow for disposing and sealing the joint surfaces between the high-pressure channel delivering the fuel from under-plunger cavity, and the high-pressure channel of the nozzle body.                According to a common formula, the maximum force that can be created by a cylindrical helical spring equals Fmax=τπd3/8D(Kgf), where τ—is maximum allowed torsion stress, d—diameter of the spring wire, D—average diameter of the spring coil. If we assume that for a cyclically working spring τ=3,000 kg/cm2, and minimum permissible ratio D/d=3 (based on the manufacturing considerations), the formula above can be reduced to: Fmax=44D2 kgf.        Considering the dimensions of the cavity where the spring is disposed (see above) and granted that D/d=3, the permissible values for the average spring diameter will be D=0.9-1.05 cm. According to the formula above, the maximum spring force in the state-f-the-art diesels is Fmax=36+48 kgf. In modem high-speed diesels, the diameter of the nozzle needle is usually 0.6 cm and the needle cross-section differential coefficient is 0.65 (the ratio of the difference between the area of the needle cross-section and the area bounded by the circumference of the bearing edge of the needle cone to the needle cross-section area). In this case (granted that Fmax=50 kgf), the fuel pressure during the needle's travel upward will be about 400 kgf/cm2, and the fuel pressure in the beginning of the closing of the needle will be 280 kgf/cm2, which is not enough, considering the maximum injection pressures specified above (2000-2500 Bar and higher).        